With the advancement in technology and high demand for production in harsh environments, there is now a demand for advanced lubricants with high-end performances. All industries strive to increase the efficiency of the equipment and reduce power consumption. “Friction” has always been a matter of concern for all process industries. For example, an increase in friction between the gear flanks lead to high wear, which is detected by several oil analysis tests such as ferrography, TAN number, etc. Every day all process industry incurs losses in energy consumption in mechanical application. The main reason for this loss is the inability of several commercial lubricants to produce a stable high strength lubricating film between the mating pairs. The most well-known additives, such as sulfur and phosphorus, were used for several years as extreme pressure (EP) additives in controlling friction and wear . The other commonly used friction modifier and EP additives are molybdenum di- sulphide, graphite, ZDDP [2-4]. These additives have been providing excellent protection to the mating surfaces; however, the present industrial scenario demands more efficient and high-performance lubricants. This brings in need of utilizing recent technologies to enhance the performance of existing lubricants. Primarily in the oil lubrication system, by introducing a stable nano solid lubricant dispersion (nano lubricants), considering the compatibility with existing lubricants. This article will briefly introduce to you the concepts of nano lubricants, particularly the aspects of nano friction modifiers and the advantages of using a nano lubricant supported with laboratory and field results.
A quick short understanding of Nano-lubrication mechanisms:
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Major questions raised at the maintenance level:
- How to reduce the temperature of bearings and gearboxes?
- How to enhance the life of the equipment?
- How to reduce the energy consumption of the equipment?
- Can a new lubrication technology be adopted?
- Can the effects of new lubricants be quantized? Etc.
To answer these questions, the introduction of the Future Lubrication needs to be understood. An oil lubricant contains about 5%-10% of different types of additives. This article will discuss more on the friction modifiers (solid lubricants). Apart from the chemical composition of the solid lubricants, the effective lubrication also depends on the sizes of the solid lubricants. The smaller the sizes of the solid lubricants, the easier for them to penetrate in the contact region between the mating pairs . Nanotechnology has attracted the attention of material scientists and practising engineers due to the exceptional improvement of the mechanical properties of nanoparticles as compared to their micro counterparts. Several researchers have reported a significant reduction in wear and power consumption using nano lubricants . When nanoparticles are added to lubricants as friction modifiers, they improve the lubricity of the lubricants by adopting either one or a combination of the following mechanisms (Figure 1):
- Nano ball-bearing effect: Spherical nanoparticles act as Nano bearings between the mating pairs. These spherical nanoparticles roll between the mating pairs and thus, separating the mating pairs.
- Mending effect: Because of the nano sizes of the particles, the nanoparticles form a fills the valleys and other surface defects and gradually forms a protective layer. This formation of the nanoparticle layers on the valleys also increases the surface areas, resulting in better load-bearing capacity.
- Formation of tribo-film due to the tribochemical reactions: Several nanoparticles also forms a protective layer on the surfaces, thus minimizing the chances of metal to metal contact.
- Polishing effect: Few nanoparticles acts on the highly stressed areas and polish the surfaces to reduce the friction between the mating pairs.
It has also been reported that several nano solid additives align themselves in the direction of lubricant flow and hence, decreases the frictional drag that is produced during the operation. It is also important to understand the morphology of the nanoparticles, which contribute to the lubrication mechanisms. The majority of the nano solid additives are spherical, thus acting as a ball bearing between the mating pairs. The sheet-like nano solid additives such as graphene, boron nitride, molybdenum disulphide etc., contribute to the lubrication due to the shearing of their adjacent planes .
An interesting morphology is the onion morphology of the nanoparticles (Figure 2). These onion-like structures are spherical when stable or exfoliate and form sheets. Inorganic fullerenes follow the rolling, sliding and exfoliating mechanisms. Thus, the lubrication mechanism also depends on the morphology of the nano additives.
Advantages of adding nano solid lubricants along with commercial lubricants:
Now a days “Micro Ceramic” Nano Technology is used in Industrial and Automotive Applications resulting in significant savings for the end-users. Researchers have indicated a significant reduction of wear (as low as 20%) on several occasions with the introduction of nanoparticles in the tribo-system . These nano solid additives make up the deficiencies of the commercial lubricants containing micro-sized additives. Field results in industrial gearboxes, reciprocating compressors, spindles in spinning mills etc., indicated a significant reduction in energy consumption 5%-18% with the introduction of Nano “Micro Ceramic” additives. A decrease in gearbox temperature was also observed by 5 degrees Celsius to 7 degrees Celcius.
The primary reason for such significant improvement in the existing lubricant performance is the introduction of nano additives in the lubricants. In-depth studies have revealed that the spherical nano Micro Ceramic additives acted as ball bearings between the mating pairs and mended the surface defects, resulting in a more high strength lubricating film between the mating pairs which was not forming before the addition of the Micro Ceramic Nano Glide Compounds. The surface analysis indicated that the roughness of the mating pairs after introducing Micro Ceramic Nano Glide compounds was less than the mating pairs, which were the commercial oil without Micro Ceramic Nano glide Compounds. The control on the roughness of the mating pair by introducing Micro Ceramic Nano Glide Compounds resulted in a higher lambda value which ultimately made the system more of Mixed Lubrication Regime rather than a Boundary Lubrication Regime, which was not present when there was no Micro Ceramic Nano Glide Compounds.
Extensive fatigue tests on bearing steel balls (AISI 52100) exhibited that the introduction of nano solid additives (Micro Ceramic Nano Glide Compounds) enhanced the L10, L50 and L90 life of the bearing steel as compared to the commercial, industrial oil (without the nano solid lubricants). As seen from Figure 3, the bearing steel balls in the present commercial gear oil 320 cSt without Micro Ceramic nano Glide Compounds exhibited high plastic deformation and surface damages compared to the bearing steel balls in the presence of Micro Ceramic Nano Glide Compounds added commercial gear oil 320 cSt. Thus, when added to the gear oil, the Micro Ceramic Nano Glide Compounds helped extend the life of the bearing steel balls, which was not possible when the gear oil without Micro Ceramic Nano Glide Compounds was used.
The application of nano solid additives is restricted to industrial applications. In automotive industries such as heavy vehicles in the mining industry and passenger car segment, nano additives have improved the engine performances, resulting in increased mileage and reduced breakdown.
To summarize, the Next Generation Lubricant in the form of Nano Lubricant can reduce the friction and wear between the mating pairs, thus leading to reduction in power consumption with Energy Efficient Lubrication towards Sustainable Lubrication Engineering. However, it is well known that if we are adding Nano additives externally to the rotating applications, a detailed oil analysis of the present oil needs to be done, as the oil serves as a carrier for the nano solid lubricants. The health of the oil must be in good condition.
- Dai, W., Kheireddin, B., Gao, H., & Liang, H. (2016). Roles of nanoparticles in oil lubrication. Tribology International, 102, 88–98.
- Xu,Z., Hu,K., Cai,Y., Huang,F, Han,C., (2014) Tribological properties of molybdenum disulphide nanoparticles in soybean oil, Tribology – Materials, Surfaces & Interfaces, 8:4, 179-186,.
- Bhaumik,S., Paleu,V, Sharma,S., Dwivedi,S., Borkar,S, Kamaraj,M, (2020) Nano and micro additivated glycerol as a promising alternative to existing non-biodegradable and skin unfriendly synthetic cutting fluids, Journal of Cleaner Production, Volume 263, 121383.
- Zhao,Y., Geng,Z., Li,D., Wang,L., Lu,Z., Zhang,G., (2021). An investigation on the tribological properties of graphene and ZDDP as additives in PAO4 oil, Diamond and Related Materials, Volume 120, 2021, 108635.
- Lee, K., Hwang, Y., Cheong, S., Choi, Y., Kwon, L., Lee, J., & Kim, S. H. (2009). Understanding the Role of Nanoparticles in Nano-oil Lubrication. Tribology Letters, 35(2), 127–131
- Bhaumik, S., Datta, S., and Pathak, S. D. (2017). “Analyses of Tribological Properties of Castor Oil With Various Carbonaceous Micro- and Nano-Friction Modifiers.” ASME. J. Tribol. November 2017; 139(6): 061802.